Uchida and Niizeki, Proceedings of the IEEE, Vol. 61, No. 8, August 1973, pages 1073-1092, and at pages 1080-1082 discuss both single crystals of tellurite and certain tellurite-containing glass compositions e.g., as materials having useful acousto-optic properties. The acousto-optic properties of a material have reference to the capability of the material to diffract incident light as an acoustic wave is propagated through the material. In the design of acousto-optic devices, it is frequently desirable to employ as the active acousto-optic element a glass composition having a high acousto-optic figure of merit, designated M.sub.2, and low acoustic loss, also referred to as acoustic attenuation.
The figure of merit, M.sub.2, represents a measurement of the inherent efficiency of light diffraction for the acousto-optic element. M.sub.2 can be calculated according to the following equation: ##EQU1## wherein
N is the refractive index,
P is the elastoptic coefficient,
V is the sound velocity in the material, and
.rho. is the density.
(The units of M.sub.2 as expressed in the "cgs" system are sec.sup.3 /gm.) In general, an M.sub.2 value of 15.times.10.sup.-18 sec.sup.3 /gm or higher is considered to represent a relatively high figure of merit for a glass composition. Therefore, glass compositions having a figure of merit in excess of 15.times.10.sup.-18 sec.sup.3 /gm are considered to represent good candidates for further evaluation as to their other acousto-optical properties, e.g., acoustic attenuation.
Acoustic attenuation for a given material is frequency dependent. In general, the acoustic attenuation coefficient, .alpha., bears an exponential relationship to the frequency of the sound wave propagated through the material. Thus, the acoustic attenuation coefficient .alpha. can be expressed by the following equation: EQU .alpha..infin.f.sup.x II
where
.alpha. is the attenuation coefficient,
f is the frequency of the sound wave propagated through the material, and
x is an empirically determined constant for each specific material.
Thus, the log of the attenuation coefficient .alpha., expressed as a function of frequency f, can be represented graphically as a straight line whose slope is equivalent to x. In general, one would desire an acousto-optic material having a value x in the neighborhood of about 1.5 or less. This would indicate the material would show a relatively small increase in acoustic attenuation as the frequency f was increased from a fairly low frequency of 50 MHz up to higher frequencies of 100 to 200 MHz or more. Operating with a minimal acoustic loss at the higher frequencies of 100 to 200 MHz or more is particularly desirable because these higher frequencies provide diffraction of incident light at a maximum angle of deflection. Acoustic attenuation figures are conveniently expressed in units of decibels per microsecond or decibels per centimeter. Thus, the acoustic attenuation coefficient, .alpha., can conveniently be measured for a material at 100 MHz and expressed in units of db/cm or db/.mu.sec.
Certain tellurite glasses have recently been described as having useful acousto-optic properties as discussed hereinabove. Thus, Masuda et al in Japanese patent application 74/100,525 published Mar. 9, 1976 describes a tellurite glass of the following composition:
TeO.sub.2 -68 mole%, ZnO-7 mole%, Li.sub.2 O-13 mole%, PbO-9 mole%, and BaO-3 mole%
as providing useful acousto-optic properties including a figure of merit in excess of 22.times.10.sup.-18 sec.sup.3 /gram, and an acoustic attenuation of 2.6 db per cm. In addition, Izumitani and Masuda have reported useful acousto-optic tellurite glass compositions composed of tellurium dioxide, tungsten trioxide, and lithium oxide. These materials reportedly had an M.sub.2 value in excess of 15.times.10.sup.-18 sec.sup.3 /gram, a value x of 1.7, and an acoustic attenuation at 100 MHz of about 3 db/cm, possibly lower. See Izumitani and Masuda, Tenth International Congress of Glass, 5, pages 74-81, published 1974.
Although the tellurite glasses described hereinabove have been though to have useful acousto-optic properties, improved tellurite glass compositions are still being sought having further improvements in acousto-optical properties, particularly improvements in acoustic attenuation with little or no decrease in the figure of merit, M.sub.2. In addition, it would be desirable to provide tellurite glass compositions which are highly transparent over the entire visible spectrum, i.e., between about 400 and 700 nm. For example, the addition of tungsten trioxide imparts a distinct yellow coloration to tellurite glasses owing to the blue absorption exhibited by tungsten trioxide.